This invention relates to foamable synthetic resin compositions containing in the thermoplastic synthetic resin an amount of 0.1 to 10% by weight of one or more compounds which have the actions of inhibiting escape of foaming agent gas from the foamed substance, of increasing the gas efficiency of the foaming agent and of preventing marked contraction of the foamed substance.
In general, thermoplastic resin foams are manufactured by incorporating a foaming agent into the resin in heat plastified form and by gasifying the foaming agent to thereby form the desired thermoplastic resin foam. Foaming agents typically employed can oftentimes be relatively expensive materials. Thus, improvement of gas efficiency of said foaming agent is of extremely great significance in industrial production.
Of the thermoplastic resins, it is considered to be especially difficult to cause foaming of polyolefin resin on an industrial basis in unaltered form. The reasons for this are thought to be that the thermal fluid viscoelastic characteristics of polyolefin resins during melt processing are markedly temperature-dependent and that the thin film or membrane forming the cell walls within the foam resin is highly permeable to gases so that it is difficult to maintain gas pressure in the foam necessary for foaming. These factors have complex effects on the swelling and heat absorption that occur during foaming and on the heat of crystallization, making it difficult to establish foaming conditions. Consequently, it is a matter of common sense that, in the foaming of polyolefin resins and particularly in foaming thereof at high foaming multiples or at high expansion ratios (e.g., greater than 5) foaming is carried out with the resin being necessarily modified by crosslinking or with the resin being modified by mixture with other resins.
By exception, a technique for extrusion foaming of non-crosslinked polyolefin resin has been announced in Published Japanese Patent Application No. 4341 of 1960. However, since this method cannot be accomplished except with relatively expensive foaming agents such as 1,2-dichlorotetrafluoroethane, it has obvious economic disadvantages. In addition, the formed body that is obtained gradually begins to contract (or shrink) over time immediately after formation with the result that a formed body of the intended density cannot be obtained without restoration (or gradual recovery) of the original dimensions. Even if the dimensions are restored (or gradually recovered), a relatively long period of time is generally required for this and furrows oftentimes remain on the surface of the formed body, giving the formed product a poor external appearance. There is the further defect that there are problems relating to dimension control that have not been solved.
This invention was developed in the light of the foregoing state of affairs and it thus has the objective of providing thermoplastic synthetic resin compositions for foaming by means of which thermoplastic resin foamed bodies can be manufactured economically, and, in the case of polyolefin resins in non-crosslinked form in particular, by means of which foam formation can be carried out easily and by which the foamed body obtained exhibits little or substantially no contraction over time and is of superior dimensional stability and surface smoothness.